[Consensus] SyncManager - import std::Result instead of failure::Result
[Consensus] SyncManager - import std::Result instead of failure::Result

This seem to make more sense, as std::Result is used more often.

aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra